This invention relates to a dynamic combustion characteristic sensor for internal combustion engines and particularly for such a sensor sensitive to combustion pressure induced strains between the block and the head of an internal combustion engine. Such a sensor is useful in generating signals for use in a system to control ignition timing on the basis of the crankshaft-related location (timing) of peak pressure (LPP) and/or on the basis of detected engine knock or detonation.
The prior art includes such dynamic sensors as engine vibration or knock sensors which are physically attached to an engine component in order to sense the vibrations transmitted through the component, engine pressure sensors physically exposed to the combustion chamber to directly measure the pressure therein and quartz force rings mounted on or under spark plugs or under the head of a headbolt. The prior art further includes a variety of static tension indicating bolts which may be accurately tightened to a predetermined preload force or torque. However, if it is desired to produce a sensor primarily for use in an LPP location controlled spark timing system which is also sensitive to engine knock and is inexpensive and easy to manufacuture in large quantities, each of the above-mentioned prior art items has some drawback. For example, direct pressure sensor and spark plug mounted quartz rings must be used one for each combustion chamber. Static tension indicating bolts of prior art construction are awkwardly constructed, expensive and difficult to interface with electric circuitry because of scaling problems.
The sensor of this invention is a modified engine headbolt of the type which holds the engine head to the engine block and is thus subject to the tensions exerted between the block and head by the changing pressures of gases within the combustion chambers. The prior art comprising a quartz force ring or washer compressed under the head of an engine headbolt to sense combustion chamber peak pressure is probably the closest prior art to the sensor of this invention. However, although a quartz ring can make a workable system in a laboratory environment it is not suitable for mass production on vehicles. The item is expensive; and it is separate from the bolt and therefore might be improperly installed. In addition, being under the bolt head, it must bear the full bolt force and must therefore be made quite large so that the head must be machined to create a space for it. Finally, it introduces a spring constant into the headbolt to engine head interface which has been seen to generate low frequency ringing in the peak cylinder pressure signal output which impedes accurate detection of the location of maximum peak pressure.
This invention therefore contemplates modifying the headbolt itself by placing a thin piezoelectric disc on the top surface of the headbolt head and surrounding that element with electrostatic shielding to prevent the contamination of the electrical signal therefrom by capacitive charges or electromagnetic signals generated by the high voltage ignition and other devices present in the engine environment. The resultant sensor is self-contained and inexpensive to produce and is easily installed on an engine in place of an ordinary headbolt. It is used with an output conditioning electrical circuit which rejects low frequency signals generated by thermal stresses between engine block and head. The details and advantages of this invention will be apparent from the accompanying drawings and following description of a preferred embodiment.